508134-01
Issue 2045
Page 14 of 53
Combustion Air Inducer (B6) & Cold End Header Box
All
GDD92C
units use a combustion air inducer to move air
through the burners and heat exchanger during heating
operation. The blower uses a shaded pole 120VAC motor.
The motor operates during all heating operation and is
controlled by integrated control A92. Blower operates
continuously while there is a call for heat. The integrated
control will not proceed with the ignition sequence until
combustion air inducer operation is sensed by the proving
switches.
The combustion air inducer is installed on the cold end
header box. The cold end header box is a single piece made
of hard plastic. The box has an internal channel where the
combustion air inducer creates negative pressure at unit
start up. The channel contains an orifice used to regulate
flow created by the combustion air inducer. The box has
pressure taps for the combustion air inducer pressure
switch hoses. The pressure switch measures the pressure
across the combustion air inducer orifice or difference in
the channel and the box. If replacement is necessary the
gaskets used to seal the box to the vestibule panel and the
combustion air inducer to the box, must also be replaced.
Combustion Air Pressure Switch (Figure 10)
GDD92C
series units are equipped with a differential
pressure switch located on the cold end header box. The
switch monitors across the combustion air inducer orifice
to insure proper flow through the heat exchanger.
The switch is a SPST N.O. pressure switch electrically
connected to the integrated control. The purpose of the
switch is to prevent burner operation if the combustion air
inducer is not moving enough air for proper combustion.
Figure 10. Pressure Switch
On start-up, the switch monitors whether the combustion
air inducer is operating. It closes a circuit to the integrated
control when the difference in pressure across the
combustion air inducer orifice exceeds a non-adjustable
factory setting. If the switch does not successfully sense
the required differential, the switch cannot close and the
furnace cannot operate. If the flue or air inlet become
obstructed during operation, the switch senses a loss of
pressure differential and opens the circuit to the integrated
control. If the condensate line is blocked, water will back
up into the header box and reduce the pressure differential
across the switch. The pressure switch opens if the
differential drops below the set point. See Table 6.
Checks of pressure differential can aid in troubleshooting.
When measuring the pressure differential, readings should
be taken at the pressure switch. See Figure 11 and Table 7.
Lack of differential usually indicates problems in the intake
or exhaust piping, but may indicate problems in the heat
exchanger, condensing coil, header boxes, combustion
inducer or other components.
Unit
Altitude (ft.)
0 - 4500
Set Point “w.c.
045
-0.65”
070
-0.65”
090
-0.65”
NOTE:
Set point is factory set and non-adjustable
Table 6.
